The prior art is replete with various transmissions for agricultural tractors and the like. Multispeed transmissions having countershafts are widely used in the power train of tractor arrangements because a plurality of rotating clutch assemblies and associated gears can be positioned on parallel shafts to allow considerable flexibility in adapting them to different space requirements and "gear spacing".
"Gear spacing" is the ratio change between gears which produces the change in vehicle speed when the operator shifts to a different gear. The smaller this gear spacing the better the optimum engine speed can be matched to the optimum ground speed. The more gear selections that are available, the finer the gear spacing can be designed. However, the number of clutches and gears increases with added gear selections, increasing the cost, etc.
"Shift quality" is the operator's perception of how smoothly a transmission reacts when making a shift. Many factors affect shift quality, such as rapid changes in speed of elements with large inertia within the transmission, poor timing of the pressure control, large torque interruptions at heavy loads, large gear spacing, and most of all, the number of clutch "swaps" required from one gear selection to the next. (A single clutch swap is defined as the disengagement of one clutch and the engagement of another clutch to complete a shift.) All currently manufactured powershift transmission have multiclutch swaps during some shifts in the operating range. It is difficult, if not impossible, to make multiclutch swaps smooth because during a shift, one or more of the engaging clutches opposes the direction of the shift. For example, in one typical transmission, during a triple clutch swap upshift from 6.sup.th to 7.sup.th gear, one of the clutches shifts up while the other two clutches shift down.
Any sequence of clutch engagements will cause torque reversals. This effect is inherent in all multiswap shifts of current designs. Only single clutch swaps can be shifted smoothly by overlapping the engagement of oncoming clutch with the disengagement of the outgoing clutch.
There are special applications for the type of vehicle using the transmission described herein where exact speed control is important, such as trenching or certain planting and harvesting operations, among others. In many of these applications, most of the engine power is used to drive the mechanism of the towed attachment through the power take off (PTO) with only part of the power used for the forward motion of the vehicle. Here, transmission efficiency is of secondary importance, with the primary importance being the ability to vary the ground speed at small increments independently of the engine. In such cases, an continuously variable transmission is desirable. Continuous variability can be achieved, for example, through the use of hydrostatic units, in which case, transmission efficiency is sacrificed. Other continuously variable transmissions include electrical generator-motor sets or variable friction drives.
Two types of the continuously variable transmission are of interest:
1. Continuous variability from a certain minimum vehicle speed to the maximum vehicle speed with full power transmission capability in one mode of operation, and continuous variability from zero to a certain low vehicle speed with maximum traction capability in another mode of operation. In the full power mode a means (such as a clutch) must be available to start the vehicle in motion at full load. As pertains to the present invention, this type will be called a partial continuously variable transmission (PCVT).
2. Continuous variability from zero to maximum vehicle speed with full power and maximum traction capability within a single mode of operation. As pertains to the present invention, this type will be called a full continuously variable transmission (FCVT).